In conventional casting of metal ribbon, filaments, fiber, or film it is often difficult to obtain and/or maintain a uniform flow of molten material across the exit or pouring lip of a tundish onto the surface of a heat-extracting substrate. This difficulty is due to, among other factors, non-uniform temperature distribution of the molten metal or metals in various parts of the tundish, or to non-uniform mixing of the molten material, or to non-uniform velocity of the molten material as it flows through the tundish toward the pouring lip.
Various systems have been employed to attempt to avoid these problems For example, U.S. Pat. No. 4,678,719, issued on July 7, 1987 to Johns et al. and assigned to Allegheny Ludlum Corporation, teaches a widening tundish in an attempt to control the velocity profile of the flow of molten material to thereby aid strip casting of crystalline metal. However, additional baffles and weirs are needed to further control the velocity profile toward the cooling substrate and/or to control the depth of the molten material across the width of the tundish. Furthermore, the design of Johns et al. provides only laminar flow or direct delivery of the molten flow to the casting or cooling surface. This can result in non-uniform delivery rate across the surface of the cooling substrate.
Hackman, et al., in U.S. Pat. No. 4,813,472, issued Mar. 21, 1989, teaches an improved method for producing filaments or fiber from a molten material by overflowing the molten material against the surface of a rotating cooling substrate.
Also known are orifice-type casting systems wherein molten material is delivered from a nozzle to the quenching or casting surface. However, poor quality can result from such casting systems due to non-uniform cooling, partial shrinkage of the strip, and the development of cracks in the strip.
In addition, orifice-type extrusion systems suffer from relative complexity of the necessary process control systems and the difficulty in passing a molten material through fixed, small orifices. The orifice must be constructed from an exotic material if the molten material has a relatively high melting point. The orifices have a tendency to erode and/or become partially or completely blocked due to the freezing of material on the orifice.
Witt, et al., U.S. Pat. No. 4,229,231, issued Oct. 21, 1980, claims a method of forming a multilayered solid structure by means of rapid quenching of separate melts on a fast moving heat extracting surface. However, Witt et al. is directed to orifice extrusion technology. Similarly, Pond et al. in U.S. Pat. No. 4,326,579, issued Apr. 27, 1982, claims an extrusion method for forming a filament from molten material.
Conventional casting methods also suffer from a lack of control over the wetting of the exit lip and/or the surface of the cooling substrate. This lack of control diminishes the width or the quality of the cast strip and often requires considerable time and effort to achieve the desired wetting of the exit lip and the cooling substrate necessary to produce acceptable cast product.
Thus a method and apparatus is desirable which is suitable for commercial production of strip at reduced cost and with improved control of the molten material flow.
It is an object of the present invention to provide a method and an apparatus for improved direct casting of metal strip, which method is superior to orifice-type casting and other known casting processes.
Another object of the present invention is to provide a system for forming ribbon, filaments, fiber, or film products directly from a molten material in a manner whereby the depth, cooling rate, wetting of the exit lip, and velocity of the molten material flowing toward a cooling substrate are controlled.